Production of heavy metal carbides of high specific surface area

ABSTRACT

A method of producing heavy metal carbides of high specific surface area characterised in that a compound in the gazeous state of said heavy metal is caused to react with reactive carbon having a specific surface area at least equal to 200 m 2 .g -1  at a temperature comprised between 900° and 1400° C., and thus obtained carbides.

This is a divisional of Application Ser. No. 07/613,627, filed Nov. 27,1990, now U.S. Pat. No. 5,308,597.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the production of metal carbides with a highspecific surface area and stable even at high temperature and theactivation of the said surface particularly when they are used as acatalyst in chemical reactions.

STATE OF THE ART

High temperature catalysis is in particular used in the petrochemicalsindustry (for example reforming . . . ), catalytic exhaust converters orany other high temperature chemical reaction.

On the one hand, it is interesting to be able to conduct catalyticreactions at the highest possible temperatures and on the other to beable economically to regenerate the catalysts with or without theircarrier for repeated uses. With such an application in mind, it isnecessary for the catalyst and/or its carrier not to be damaged whenthey are subjected to high temperatures. From this point of view,aluminina with a large surface area is not satisfactory. That is why theApplicants have already perfected a process for obtaining a catalystcarrier consisting of silicon carbide with a high specific surface area,doped or otherwise, on which the catalyst element is deposited (EP88-420352-2).

Continuing their research, they then sought to obtain carbides of metalssuch as, for example, Mo, W, Ro, V, Nb, Ta, Ti . . . of high specificsurface area, possibly in excess of 200 sq.m/g. These carbides likewiseoffer considerable interest as a high temperature catalyst carrier butin contrast to a non-metallic element such as Si, they exhibit catalyticproperties as such, or they may exacerbate or modify the properties ofthe catalytic active phases (for example of Pt) which are depositedthereon. It is quite obvious that carbide powders having such a surfacearea may likewise be used for other applications such as, for example,the production of sintered parts having unprecedented properties, bymeans of simplified processes.

A process is already known (Journal of Catalysis Vol. 106, pp. 125-133,1987) for obtaining Mo₂ C carbide of a specific area which does notexceed 100 sq.m/g, on the basis of a solid MoO₃ oxide and a CH₄ :H₂gaseous mixture according to a temperature programmed reaction whichmust take into account the value of the H₂ /CH₄ ratio, a reaction whichtherefore calls for special attention if one is to avoid the formationof polymeric carbon which is a catalyst contaminant and which mightpossibly have to be destroyed by a special process. Furthermore, thisfairly complex method does not lend itself readily to industrialproduction of a non-contaminated product.

Furthermore, the document Journal of Catalysis Vol. 112, pp. 44-53,1988, while pointing out the need to have Mo₂ C carbide which has aspecific surface area of more than 200 sq.m/g, suggests a method ofobtaining purely an alpha-MoC_(1-x) carbide with a specific surface areagreater than 200 sq.m/g. This method is also more complex than thatpreviously mentioned since it passes through an intermediate stage ofproducing a nitride which is then converted to the carbide likewise by atemperature programmed reaction.

OBJECTS OF THE INVENTION

Faced with these difficulties, the Applicants have sought to perfect asimple process which could be used for large scale production of metalcarbides of high specific surface area, that is to say always greaterthan 20 sq.m/g or better than 50 sq.m/g but generally far greater than100 sq.m/g, such a carbide possibly being in the form of granules orshaped pieces.

Another object of the invention is to provide heavy metal carbides whichcan be used preferably either as a catalyst carrier or as a catalystused as such or deposited on a carrier; other applications of thesecarbides are likewise possible in other fields, for example sintering.

Another object is to obtain mixed carbides of heavy metals of highspecific surface area which may offer significant advantages if used incatalysis. Thus it is possible to obtain shaped pieces or grains havinga core of a different chemical nature from that of the periphery of thesaid mouldings or grains. Thus, these carbides can act either as acatalyst carrier or as an active catalyst.

DESCRIPTION OF THE INVENTION

The invention is a method of producing heavy metal carbides with a highspecific surface area, characterised in that a compound of the saidheavy metal is, while in the gaseous state, caused to react withreactive carbon having a specific surface area at least equal to 200sq.m/g at a temperature of between 900° and 1400° C.

It is particularly suitable for the production of large granules orshaped pieces, transformation of the carbon to carbide being carried outover a thickness which may be as much as 2 mm.

The metal objects according to the invention are preferably transitionmetals (groups 3b, 4b, 5b, 6b, 7b, 8 of the series or periods 4, 5, 6),rare earths (lanthanides) and the actinides.

Furthermore, the said metals are obviously those which have a compoundwhich is volatile at least under the temperature and pressure conditionsof the reaction. Among these compounds, preferably chosen are the oxidesor the iodides, but other compounds may be used, for example chlorides,fluorides or certain organometallic sublimable complexes. Among themetals which have this characteristic, those which are worthy of specialattention for the catalytic interest of their carbides are metals suchas Mo, W, R, V, Nb, or even Ta, Ti, Cr, Ni.

The method consists in causing the metallic compound in gaseous form toreact with the reactive carbon of large surface area.

It is important that the gaseous metallic compound directly attack thereactive carbon which has the large specific surface area so that it ispossible to retain a memory of the said large surface area in theresultant carbide with the surface properties needed for catalyticapplications.

For this, it is sometimes advantageous at the start and in the reactioncontainer to separate on the one hand the metallic compound and on theother the reactive carbon so that during the thermal treatment thegaseous metallic compound enters into reaction with the reactive carbon.Thus, it is possible to introduce into the reactor a layer of solidmetallic compound, possibly covering it with a carbon felt and thendispose one or more shaped pieces or a layer of reactive carbon grainsdirectly on the layer of compound or on the felt and then carry out aheat treatment under conditions such that the solid compound becomesgaseous and then reacts with the carbon.

It is preferable for gasification of the volatile compound to take placeas close as possible to the reactive carbon so that the reaction contactis rapid. But the gaseous current may be generated in a separateevaporation zone.

In certain cases, generally determined by the chemical nature of themetal, it is likewise possible at the outset to bring the metalliccompound and the reactive carbon in contact by an approximate or coarsemixture.

The reaction is performed at a temperature comprised between 900° and1400° C. But it is advantageous to work at the lowest possibletemperature, watching however to see that the reaction velocity issufficiently high and so it is preferable to work at between 1000° and1250° C. Beyond this temperature for the majority of carbides andparticularly for molybdenum and tungsten carbides, a notabledeterioration in the specific surface area obtained can be noted.

The reaction takes place under reduced pressure, normally less than 13hPa (10 mm Hg). It is important that the partial pressure of the gaseousmetallic compound should be as high as possible, the residual pressurebeing due mainly to CO produced by the reaction.

The reactive carbon subjected to reaction must have a BET specificsurface area of at least 200 sq.m/g and it may be used in the form of apowder or as preformed pieces, for example by agglomeration of powdersor by extrusion of a resin followed by a carbonising process.

Any type of active carbon, especially graphite, charcoal, carbon black,may be used but it is preferable to use granulated or agglomeratedactive carbon obtained, for instance, from decomposed resins orvegetable fibres.

The said carbon may be doped by a metallic element such as Ce, T, U, Zr,Hf, lanthanide . . . , by impregnation by means of a solution, aqueousor other, of a soluble compound (such as acetyl acetonate, nitrate . . .) decomposed by a heat treatment prior to the method of obtaining theheavy metal carbide according to the invention. The specific surfacearea of the carbon is then somewhat less reduced during the dopingtreatment but it does generally remain higher than 200 sq.m/g and therate of conversion of heavy metal carbide according to the invention mayaccording to the doping element be substantially increased, the dopingbeing of the order of 4 to 10% by weight of metal.

This doping may also give rise to modifications in the electronicstructure of the carbide according to the invention, investing it withnew catalytic properties.

The quantity of reactive carbon may be in excess in relation to thestoichiometry of the reaction for forming the heavy metal carbide, areaction of the type: carbon+metallic oxide→carbon monoxide+metalliccarbide.

The molar excess is normally at least greater than three times the saidstoichiometric quantity and is preferably five times greater, which istranslated for example, in the case of obtaining Mo₂ C and WC, in C/Mmolar ratios (M representing the metal) of 3/1 and 5/1 respectively. Ifthe excess is less than the low limit the appearance of a metallic phasemay greatly reduce the specific surface area of the carbide obtained.Similarly, if the temperature used is in the lower part of the rangedescribed, then it is worth while increasing the carbon excess (and thusthe C/M ratio) to avoid the appearance of the metallic phase.

The rate of conversion of the active carbon is preferably between 40 and100% but is preferably around 75% to have an excellent purity level inthe carbide (absence of metal) and a vastly improved mechanicalstrength.

The invention can be carried out in various ways. Generally, a graphitecrucible is used, closed by a simply fitted carbon felt cover which canbe installed in an enclosure (for example a tube) of silica lined on theinside with a carbon felt and closed by a carbon felt cover; the tube isenclosed with a heating means and the whole is enclosed in a cooledsealing-tight casing inside which it is possible to establish a vacuum.

It is then possible to introduce into the crucible a mixture of reactivecarbon powder, previously doped or not, and the solid compound of theheavy metal; having established the vacuum, it is heated to the selectedreaction temperature at which the said compound must be gaseous and itis left at that temperature for the time it takes for the carburisationreaction to take place, normally from 1 to 7 hours. It is likewisepossible firstly to introduce a layer of solid compound and then,covering it, a layer of reactive carbon, or a shaped piece ofagglomerated active carbon, and then to heat as previously, once thevacuum has been established. A carbon felt may possibly be interposedbetween the carbon and the compound.

It is also possible to gasify the volatile compound at a temperatureother than the reaction temperature. For this, the solid compound to bevolatilised may be installed at the bottom of the crucible and thecarbon may rest on a carbon felt situated at a distance from the saidcompound by means of spacers, heating of the bottom part and of the toppart being carried out at different temperatures, for example usingdifferent heating densities of heating windings, etc.

The method according to the invention thus makes it possible to obtaincarbides with a large specific surface area which is in excess of notonly 100 sq.m/g, as has already been stated, but easily 200 sq.m/g oreven 300 sq.m/g. These products, which can be obtained in largequantities, are easily handled and perfectly homogeneous even if at theoutset the reagents were separated or were heterogeneously blended. Thusthey can be used directly as an active catalyst, doped or otherwise,capable of regeneration but equally useful as a catalyst carrier.

According to a first improvement, the invention likewise makes itpossible to obtain mixed carbides of high specific surface area. Theterm mixed carbides is understood to mean shaped pieces, granules . . .comprising a core covered by one or a plurality of successive outerlayers of different types; the core may be of reactive carbon, as hasbeen seen previously, or may consist of some other material of highspecific surface area. This material may be either a metallic carbideaccording to the invention or a silicon carbide according to EuropeanPatent Application EP 88-420352-2, or any other oxide carrier such assilica, alumina, and the outer layer or layers are of metallic carbideof high specific surface area according to the invention, other thanthat of the core and different from one layer to the other. For example,the core may act as a catalyst carrier, the catalyst then consisting ofthe metallic carbide according to the invention which is used for theouter layer; this improvement makes it possible to limit consumption ofthe expensive gaseous metallic compound while retaining the catalyticproperties of the mixed carbide obtained, but also to modify itsmechanical properties as and when we wish.

A first variation on this improvement resides in partially reacting theheavy metal compound in the gaseous state with reactive carbon having aspecific surface area at least equal to 200 sq.m/g at a temperature ofbetween 900° C. and 1400° C., limiting the rate of conversion of thecarbon into metallic carbide in order incompletely to convert the saidcarbon. Generally, the rate of conversion is limited to a level below40% and preferably between about 10 and 20%; thus, an intermediatecarbide-carbon compound is obtained (referred to likewise as a mixedcarbide) comprising an outer layer of heavy metal carbide of highspecific surface area, according to the invention, which coats thereactive carbon core.

Such a configuration makes it possible to obtain a carbide-carboncompound having reinforced mechanical properties in the event of themetallic carbide in itself having mechanical characteristics inferior tothose of the reactive carbon.

To limit the rate of conversion of the reactive carbon, it is possiblealternatively or in combination either to reduce the quantity of gaseouscompound used in the reaction or to limit the duration of the saidreaction, the other working conditions and the characteristics of theproducts employed being the same as those described earlier in order toavoid the formation of metal as an impurity.

In order, for example, to reinforce further the solidity of thesecarbide-carbon compounds, a second alternative version of the saidimprovement shows that in a first stage it is possible to transform thecarbon core to a different metalloid or metal carbide in order to obtaina new sort of mixed carbide by reaction with a compound of the saidmetal or metalloid in the gaseous state.

During the course of this new stage, it is advantageous to convert thecore into beta-SiC according to the method of European PatentApplication EP 88-420352-2, in which SiO vapours are generated which arethen brought into contact with the reactive carbon for conversion toSiC, here the carbide-carbon compound previously obtained, at atemperature of between 1100° and 1400° C.

In this case, according to the invention, a mixed carbide is obtained inthe form of particles or pieces, comprising an outer layer of heavymetal carbide of large surface area and a silicon carbide core, normallywith a large surface area, the said mixed carbide especially havingimproved solidity.

In the event of the outer layer being of molybdenum carbide, there islikewise an advantageous presence of molybdenum silicide.

But it is also possible during the course of the said new stage toobtain a carbide of another heavy metal by the method according to thepresent invention which is described earlier. In this case, the mixedcarbide obtained comprises two different carbides of heavy metal whichconstitute on the one hand the outer layer and on the other the core, inorder to meet the needs of certain particular applications.

The metals and their compounds which are used for transforming thecarbon core into carbide may be chosen from among the same categories asthose used for obtaining the outer layer, that is to say on the one handthe transition metals (3b, 4b, 5b, 6b, 7b, 8 of the series 4-5-6 of theperiodic classification of elements), the rare earths or the actinidesand on the other the oxides and the iodides.

These carbide-carbon compounds or mixed carbides, particularly that witha SiC core of high specific surface area and of improved solidity areparticularly suitable for use as catalyst carriers or even directly as acatalyst in high temperature chemical reactions, particularly in thepetrochemicals field (reforming, cracking, oxidation, etc. . . . ),catalytic exhaust converters, etc.

On the other hand, in view of a second improvement, the Applicants havedemonstrated that after the preparation of heavy metal carbides of highspecific surface area (mixed or not), their handling in the air mayresult in a superficial alteration of the said carbide.

Such a phenomenon occurs mainly with the oxidisable carbides and givesrise to the presence of superficial oxides which in particular affectthe catalytic activity of the carbide. It is therefore important to beable to eliminate these oxides; this operation has to be carried out insitu, that is to say after the carbide of large surface area has beenplaced in the position where it has to be used as a catalyst, so thatafter having eliminated the oxides, the said catalyst is no longer incontact with the air or an oxidising atmosphere.

To eliminate the oxide, it is known to carry out a reduction by hydrogenat 800° C.; but with such an operation on the one hand there is nocomplete reduction of the oxide and on the other metal is formed so thatthe specific activity of the carbide catalyst is not regenerated: thecatalyst obtained has a different selectivity and a diminishedefficiency.

It is likewise possible to use a hydrogen-hydrocarbon mixture (forexample pentane) at 700° C.; this is the normal method of preparingcarbides; in this case, it is true that one does indeed obtain a carbidebut also a deposit of polymeric carbon which contaminates the catalyst.This carbon deposit can only be partly eliminated by a treatment withhydrogen at 600° C. and a latent contamination persists; this latter is,for example, fairly substantial with WC but a little less substantialwith Mo₂ C which catalyses this elimination in the form of ahydrocarbon.

Faced with these problems, the Applicants have, according to the saidsecond improvement, found a process which makes it possible to improvethe catalytic activity of the surface area of the catalysts whichconsist of heavy metal carbides of high specific surface area altered bya film of oxide, the said carbides being either simple carbides or anassembly of mixed carbides previously described. This method ischaracterised in that the said carbides are impregnated by means of asufficiently dilute solution of a salt of at least one metal of thegroup 8, such as Pt, Pd, Rh, Co . . . in order to have an impregnatedcarbide comprising a little of the said metal, the impregnated carbidebeing dried and treated at a high temperature under a current ofhydrogen and gaseous hydrocarbon.

It is advantageous to supplement the method by a finishing treatmentconsisting in passing over the resultant catalyst a flow of H₂ at hightemperature, generally comprised between 300° C. and 700° and preferablybetween 400° and 600° C. for 1 to 20 hours approx., possibly with one ormore intermediate temperature steps. This treatment may likewisecomprise the prior establishment of a vacuum for 1 to 3 hours at nearly350° to 500° C.

This improvement resides particularly in dispensing with the oxide oroxycarbide layer which temporarily passivated the surface of the carbidewhen it is brought in contact with the air after synthesis, without anysubstantial formation of polymeric carbon.

In the case of Pt, it is particularly important that the quantity of Ptretained by the impregnated carbide counted in percentage by weight ofPt in relation to the total mass of carbide treated should be limited.It is sufficient generally to use contents of at most 0.05% approx. andpreferably in excess of 0.001%. Contents of 0.25% currently used onalumina carriers or with a view to obtaining heavy metal carbides arenot appropriate within the framework of the present invention, becausethe activity of the catalyst will be that of the Pt and not that of thecarbide.

The salt solution of the said metal of group 8 may be achieved by meansof any solvent which can be eliminated by heating, for example water,alcohol, other sufficiently volatile organic solvents . . . Similarly,it is sufficient to choose a metal compound soluble in one of thesesolvents which does not leave any solid residue other than the metalafter high temperature treatment.

Drying is generally carried out in the air and then in an oven at atemperature which is generally comprised between 120° and 140° C.

The H₂ hydrocarbon mixture may have any proportions provided that thenecessary quantity of constituent elements is available to carry outreduction and carburisation of the oxide film. Generally the hydrocarbonis used in proportions (by volume) of 1 to 50%. It is advantageous touse this mixture in excess and to recycle the excess after havingeliminated the water produced in it during the course of reaction, forexample by condensation.

The hydrocarbon is preferably aliphatic, for example pentane or amixture of hydrocarbons.

Generally, the high temperature treatment is carried out at temperaturesabove 500° C. or preferably 700° C. It is usually easier to work atatmospheric pressure; the use of other pressures is possible.

The said second improvement thus makes it possible to obtain a catalystbased on heavy metal carbides having an isomerisation efficiency whichis better than that of a conventional Pt catalyst consisting, forexample, of 0.25% Pt on an alumina carrier used under the sametemperature conditions. It is important to note that this selectivity isdue and is peculiar to carbides prepared in this way and is entirelydifferent from the selectivity of the Pt. Particularly with tungstencarbide it is possible under certain conditions to develop substantialaromatisation, which is a favourable bonus.

After use and exhaustion of the catalyst, its regeneration mayadvantageously be carried out by treatment in hydrogen at 600° C., anoperation which is fairly remote from conventional oxidisingregeneration but far simpler to carry out, particularly in an industrialenvironment.

The catalysts and catalyst carriers according to the invention may beused in all the high temperature chemical reactions and particularly inthe petrochemicals field (cracking, reforming . . . ) or in catalyticexhaust converters for internal combustion engines.

The application of heavy metal carbides of high specific surface areaaccording to the invention to the catalytic conversion of exhaust gasesfrom internal combustion engines is particularly interesting. Especiallywith the gases from exhaust pots of motor vehicles, the said carbidesare used and are effective at a temperature higher than 200° C. or moregenerally around 350° C. or more.

These carbides, used as a catalyst, make it possible according to thisapplication completely to convert the exhaust gases: NOx into nitrogen,carbon monoxide into dioxide, hydrocarbons into CO₂ and water.

It is possible to use as heavy metal carbides the simple carbides andespecially Mo₂ C, WC . . . but also mixed carbides, especially thosecontaining an SiC core or a core of reactive carbon covered with a heavymetal carbide of high specific surface area, for example of the type Mo₂C, WC . . . .

It is advantageous to carry out the conversion reaction in the presenceof the metallic carbide catalyst, to perform an activation of thesurface of the said carbide according to the second improvementdescribed hereinabove. Let us remember that this activationsubstantially comprises impregnation of the metallic carbide with asmall quantity of a salt in a solution of a group 8 metal of theperiodic classification of elements (especially Pt), drying, heattreatment at high temperature in the presence of H₂ and gaseoushydrocarbide and possibly a finishing treatment at high temperature inthe presence of H2; the thermal treatments are carried out generallyafter having installed the carbide at the place where the catalyticconversion reaction is to be carried out.

It may likewise be advantageous to modify the active surface area of thecarbide according to the invention by partially nitriding and oxidisingit, the carbide, doped with nitride and oxide thus obtained possiblyhaving special catalytic properties.

Regeneration after use of carbides with a very large specific surfacearea according to the invention used as a carrier or as a catalyst maybe carried out simply, for example, by thermal treatment with hydrogenas has already been stated.

These carbides of high specific surface area may likewise open up theway to fresh applications in fields such as the sintering of carbides,surface coatings using carbides, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of activity versus type of catalyst for a MO₂ Ccatalyst; and

FIG. 2 is a graph of activity versus type of catalyst for a WC catalyst.

EXAMPLES Example 1

This example illustrates the obtaining of a molybdenum carbide with ahigh specific surface area according to the reaction: 2 MoO₃ (vapour)+C(solid)→Mo₂ C+3 CO.

In a first series of tests, a summary mixture of active carbon wasprepared in particles of 0.2 to 0.5 mm having a specific surface area ofabout 1150 sq.m/g and a solid oxide powder MoO₃. This mixture wasintroduced into a graphite reactor as seen earlier. The enclosure wasplaced under a vacuum of 0.13 hPa (0.1 mm Hg) prior to heating. The mainworking conditions and the results of specific surface area obtained byBET measurement are set out in Table 1.

                                      TABLE 1                                     __________________________________________________________________________       Amount of                                                                           Amount of             Specific                                                                             Carbon                                     carbon                                                                              Mo 03 Reaction        surface area                                                                         conversion                                                                            Mass                               involved                                                                            involved                                                                            temperature                                                                          Duration                                                                           C/Mo                                                                              of carbide                                                                           (including CO)                                                                        recovered                       Test                                                                             g     g     °C.                                                                           hours                                                                              molar                                                                             m.sup.2 /g                                                                           %       g                               __________________________________________________________________________    4  1.058 2.169 1155   31/2 6/1 213    50%     1.716                           5  1.144 2.614 1140   31/2 5/1 191    64%     2.033                           7  1.141 3.646 1160   31/2 4/1  89    76%     2.588                           8  1.203 4.734 1155   31/2 3/1  40    100%    2.854                           9  1.213 7.278 1155   31/2 2/1  2     100%    2.918                           __________________________________________________________________________

Test 9 illustrates that when the carbon excess is insufficient, the highspecific surface area cannot be obtained.

The rate of conversion is calculated in relation to the amount of carbonat the start, taking into account the CO which is formed.

All the products have a very homogeneous appearance.

The main product obtained is hexagonal Mo₂ C sometimes with traces ofcubic Mo₂, gamma MoC and/or metal.

Thus, hexagonal carbide Mo₂ C is obtained which has a specific surfacearea of more than 100 sq.m/g in tests 4 and 5 and 200 sq.m/g in test 4and although the specific surface area is high in tests 7 and 8, one cansee that it is markedly below that of tests 4 and 5. This is due to thepresence of a fairly substantial metallic phase and illustrates howimportant it is to work with a considerable carbon excess.

A second series of tests was carried out by the same method to test theeffect of the reaction temperature; the tests are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________       Amount of                                                                           Amount of             Specific sur-                                                                        Carbon                                     carbon                                                                              Mo 03 Reaction        face area of                                                                         conversion                                                                            Mass                               involved                                                                            involved                                                                            temperature                                                                          Duration                                                                           C/Mo                                                                              the carbide                                                                          (including CO)                                                                        recovered                       Test                                                                             g     g     °C.                                                                           hours                                                                              molar                                                                             m.sup.2 /g                                                                           %       g                               __________________________________________________________________________    14 1.005 2.016 1210   4    6/1 139    50%     1.442                           15 1.208 2.781 1170   4    5.2/1                                                                             159    58%     2.09                            16 0.987 2.013 1150   4    6/1 202    51%     1.596                           17 1.227 2.157 1120   4    6.8/1                                                                             261    47%     1.851                           18 1.001 2.024 1065   4    6/1 239    51%     1.465                           __________________________________________________________________________

In these tests, the product obtained is the same as before: hexagonalMo₂ C sometimes with traces of cubic Mo₂ C, gamma MoC and/or Mo metal.

It can be seen that, everything else being equal, the greater specificsurface areas are obtained with the lesser temperatures.

A third series of tests was carried out by initially separating theactive carbon particles from the solid powdered subjacent Mo03 by acarbon felt in order to test the effect of the reaction time. The otherreaction parameters were the same as before. The results are given inTable 3.

                                      TABLE 3                                     __________________________________________________________________________       Amount of                                                                           Amount of             Specific                                                                             Carbon                                     carbon                                                                              Mo 03 Reaction        surface area                                                                         conversion                                                                            Residual.sup.Mo                    involved                                                                            involved                                                                            temperature                                                                          Duration                                                                           C/Mo                                                                              of carbide                                                                           (CO included)                                                                         (metal)                         Test                                                                             g     g     °C.                                                                           hours                                                                              molar                                                                             m.sup.2 /g                                                                           %       %                               __________________________________________________________________________    49 0,553 1.309 1210   4    3,5 196    46      1                               50 0,564 1,354 2210   5    3,5 181    51      0,5                             59 1,184 2,798 1210   6     3,5.                                                                             176    57      0                               46 1,134 2,614 1210   8    3,5 168    64      0                               __________________________________________________________________________

In this Table, it can be seen that it is worth while increasing thereaction time in order to avoid the presence of metallic Mo (measured byquantitative diffraction X).

Example 2

This example illustrates the production of tungsten carbide with a largespecific surface area according to the reaction: 3 WO₂ (vapour)+8 Csolid→WC+W₂ C+6 CO.

The same production pattern was followed as is shown in Example 1, theactive charcoal having been roughly mixed with solid oxide WO₂. Theresults obtained are set out in Table 4 together with the principleoperating conditions.

                                      TABLE 4                                     __________________________________________________________________________       Amount of                                                                           Amount of             Specific                                                                             Carbon                                     carbon                                                                              WO2   Reaction        surface area                                                                         conversion                                                                            Mass                               involved                                                                            involved                                                                            temperature                                                                          Duration                                                                           C/W of carbide                                                                           (CO included)                                                                         recovered                       Test                                                                             g     g     °C.                                                                           hours                                                                              molar                                                                             m.sup.2 /g                                                                           %       g                               __________________________________________________________________________    23 1.607 3.946 1155   4    7.3/1                                                                             394    --      1535                            25 1.614 4.044 1175   5    7.3/1                                                                             365    51%     1.183                           26 1.194 4.275 1190   4    5/1 350    45%     1.216                           27 1.501 5.123 1185   3    5/1 294    73%     0.726                              doped 3.1%                                                                    Ce                                                                         29 1.01  4.61  1203   3    4/1 386    57%     0.621                           __________________________________________________________________________

The carbon conversion rate is in Example 2 about 50%. The productobtained is hexagonal WC with a specific surface area greater than 300sq.m/g in tests 23-25-26-29 and with traces of W₂ C and metallictungsten.

On the other hand, test 27 in which the carbon was doped with Ce showedin equivalent quantity hexagonal WC and alpha-hexagonal W₂ C and aspecific surface area which is a little smaller but close to 300 sq.m/g.

The specific surface areas are particularly high.

Example 3

This example is similar to Examples 1 and 2 and illustrates theproduction of a vanadium carbide with a high specific surface areaaccording to the reaction: V₂ O₅ (gas)+7C (solid)→2VC+5CO.

The same production procedure was followed as in Example 1, the activecharcoal having been roughly mixed with solid oxide V₂ O₅. The workingconditions and results are as follows:

    ______________________________________                                        Test No. 34                                                                   ______________________________________                                        Amount of carbon involved                                                                             0.800 g                                               Amount of V.sub.2 O.sub.5 involved                                                                    1.001 g                                               Reaction temperature    1196° C.                                       Reaction duration       4 hours                                               Molar C/V ratio         6/1                                                   Specific surface area of the carbide                                                                  235 sq. m/g                                           Carbon conversion       57%                                                   (CO included)                                                                 ______________________________________                                    

X-ray analysis shows the presence of VC and gamma VC.

Example 4

In situ reactivation tests were carried out according to the secondimprovement on tungsten and molybdenum carbides with a high specificsurface area obtained in the previous examples.

These carbides were impregnated by means of an aqueous solution of H₂ PtCl₆ so that after drying in an oven at 135° C., a quantity of Pt wasobtained amounting to 0.05% in relation to the total mass of carbide.Heat treatment was carried out at 700° C. at atmospheric pressure, witha mixture of hydrogen and pentane (2.7 kPa, or 20 mm mercury, partialpressure).

This was followed by a finishing treatment establishing a vacuum at 400°C. for 1 hour followed by treatment with H₂ at 400° C. for 14 hours andthen at 600° C. for 2 hours.

The activity of the reactivated carbides obtained was monitored bymeasuring their selectivity and efficiency for a reforming type ofreaction: isomerisation of the methyl cyclopentane (MCP) and of then-hexane (HEX). The principle consists in passing a gaseous organiccompound-here of the type (C6)-over the catalyst under review and inmeasuring at a first stage its rate of conversion representing the totalof isomeric and cracked products obtained, and in then noting therespective proportions of isomeric products and cracked products(counted in a C6 equivalent according to the example) obtained, knowingthat what is the most interesting within the framework of the presentinvention concerns the selectivity in order to obtain isomerisationpreferably and finally in detailing the results of the isomerisationitself (speed, efficiency)

In the case of the Mo₂ C catalyst of the present example, conversioncomparison of MCP and HEX have been carried out with any Mo₂ C: nonreactivated, reactivated by means of H₂ alone at 800° C. or by means ofthe mixture H₂ +pentane at 700° C. (followed by treatment in a vacuum at400° C. for 1 hour and then under H₂ at 400° C. for 14 hours and at 600°C. for 2 hours in order to eliminate the polymeric carbon), andreactivated according to the second improvement as has already beendescribed.

In the case of WC, the comparison with non-reactivated WC does notappear but there is on the other hand an isomerisation comparisonobtained in the presence of a conventional catalyst consisting of agamma alumina support impregnated with 0.25% Pt.

The comparative reactions of isomerisation in the presence of thevarious catalysts have been carried out under the same workingconditions. However, the temperatures are not always the same and it isimportant to note this having regard to the fact that this parameter hasa considerable influence on the rate of isomerisation and therefore onits efficiency; in particular in the case of carbide reactivated by H₂alone, the isomerisation temperature is markedly higher (600° C.) andthis sole parameter multiplies the the said velocity by several ordersof magnitude which may attain 10⁴ to 10⁶ in relation to a temperature of350° C.

Tables 5 and 5a therefore show the comparative results of isomerisationof MCP and of HEX obtained with a carbide Mo₂ C catalyst with a highspecific surface area reactivated in various ways.

Tables 6 and 6a likewise summarise similar results obtained with WCreactivated in various ways and with Al₂ O₃ +0.25% Pt.

In these tables:

% conversion: represents the molar percentage of starting productconverted by isomerisation and cracking.

r: represents the rate of conversion expressed as 10⁻¹⁰ moles ofstarting product converted per second and in relation to 1 g ofcatalyst. This represents the activity of the catalyst.

Si %: represents selectivity, that is to say the molar proportion ofisomerised product present in the converted product obtained (expressedin C6).

% cracking: represents the molar proportion of cracked product presentin the converted product obtained (expressed in C6).

The sum (Si+cracking) is therefore equal to 100.

Isomerisation efficiency (10⁻¹⁰ mol/s.g.): this is the product (r×Si)expressed in 10⁻¹⁰ mole isomers obtained per second and related to 1 gof catalyst.

Isomers in C6 (%): represents the detail of the isomers obtainedexpressed in molar percent (DM-2,2β=dimethyl 2,2-butane, M2P=methyl 2pentane, M3P=methyl 3 pentane, HEX=n-hexane, MCP=methyl cyclopentane,BEN=benzene, CYC=cyclohexane). The total is equal to 100.

Hydrocarbons (%): represents the detail of the cracked productsobtained, shown in C₆ and expressed in molar percent. The total is equalto 100.

In each of FIGS. 1 and 2 which relate respectively to the catalysts, Mo₂C and WC, there is a comparison of the activity (or rate ofisomerisation) of catalyst treated by H₂ +pentane and according to theinvention by H₂ +pentane+0.05% Pt in isomerisation reactions ofdifferent compounds at 350° C.

In these figures are shown the values of r given in the previous tablesconcerning HEX and MCP and the values of r when M2P and M3P are thestarting hydrocarbons.

Tables 5 and 5a relate to Mo₂ C and clearly show that the isomerisationrates r, or rather the isomerisation performances, are markedly improvedwith the catalyst which is activated according to the present inventioncompared with what it was possible to obtain with activations carriedout according to the prior art. This comparison is valid even withcatalysts activated with hydrogen alone; indeed, the conversion wascarried out in this case at 600° C. and if such a temperature had beenused for a catalyst activated according to the invention, an efficiencywould have been obtained which would be rather more situated towards10,000-100,000.

In FIG. 1 which sets out the results and expands them to other products(M2P and M3P) it can be clearly seen that the isomerisation speedobtained with the activated carbide according to the invention is morethan three times greater than that obtained with a carbide activatedonly by H₂ +pentane without Pt.

In Tables 6 and 6a which relate to WC the same results are noted asbefore which therefore give a marked advantage to the carbidereactivated according to the present invention. Furthermore, this latteris compared with a conventional catalyst in which the active element isPt deposited on a gamma alumina carrier; this illustrates again theadvantage of the heavy metal carbide of high specific surface areaactivated by means of H₂ +pentane in the presence of a small quantity ofPt in improving the speed and efficiency of isomerisation; but it islikewise noted that the selectivity of the reactivated carbide accordingto the invention is completely different from that of Pt and thereforethat the Pt present in the carbide acts at the level of activation ofthe said carbide but not as a catalyst for the conversion ofhydrocarbon.

                  TABLE 5                                                         ______________________________________                                        Isomerisation of MCP on the catalyst Mo2C                                     reactivated in various ways                                                            Prior art       Invention With                                                  Not      With    With   H2 +                                       Reactivation                                                                             reactiva-                                                                              H2      H2 +   pentane +                                  of the carbide                                                                           ted      alone   pentane                                                                              0.05% Pt                                   ______________________________________                                        Conversion 400      600     350    350                                        temperature °C.                                                        Conversion %                                                                             0,36     14,94   1,88   8,03                                       r. (10 exp -10                                                                           53       1005    167    642                                        mole/s.g)                                                                     Si %       72       45      55     60                                         cracking % 28       55      45     40                                         Isomerisation                                                                            38       452     92     385                                        output                                                                        (r × Si)                                                                Isomers in C6 %                                                               DM-2,2B    --       --      --     --                                         M2P        28       13      37     51                                         M3P        11       0,5     15     26                                         HEX        33       1,5     13     12                                         MCP        --       --      --     --                                         BEN        --       85      29     11                                         CYC        28       --       6     --                                         Hydrocarbons %                                                                C5 + C1    44        3      38     32                                         C4 + C2    33       12      26     27                                         2C3        11       13      16     18                                         3C2        --       15       7      8                                         6C1        12       57      14     15                                         ______________________________________                                    

                  TABLE 5a                                                        ______________________________________                                        Isomerisation of n-hexane on catalyst M02C                                    reactivated in various ways                                                            Prior art    Invention                                               Reactivation                                                                             With H2  With H2 + With H2 + pentane +                             of the carbide                                                                           alone    pentane   0.05% Pt                                        ______________________________________                                        Conversion °C.                                                                    600      350       350                                             temperature                                                                   Conversion %                                                                             40,13    4,16      14,93                                           r. (10 exp -10                                                                           2497     362       1283                                            mole/s.g)                                                                     Si %       26        9        14                                              cracking % 74       91        86                                              Isomerisation                                                                            649      33        180                                             output                                                                        (r × Si)                                                                Isomers in C6 %                                                               DM-2,2B    --       --        --                                              M2P         3       44        43                                              M3P         1       25        23                                              HEX        --       --        --                                              MCP         1       12        17                                              BEN        56       19        17                                              CYC        39       --        --                                              Hydrocarbons %                                                                C5 + C1     3       30        29                                              C4 + C2    15       36        32                                              2C3        25       24        26                                              3C2        31        5         7                                              6Cl        26        5         6                                              ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Isomerisation of MCP on catalyst WC                                           reactivated in various ways                                                                         Invention                                                        Prior art    with       Al2O3                                        Reactivation of                                                                          with H2  with H2 + H2 + pentane                                                                           0.25%                                  the carbide                                                                              alone    pentane   0.05% Pt.                                                                              de Pt                                  ______________________________________                                        Conversion 600      450       450  350   350                                  temperature %                                                                 Conversion %                                                                             13,10    2,32      8,08*                                                                              12,12*                                                                              2,82                                 r. (10 exp -10                                                                           745      178       1680 945   114                                  mole/s.g)                                                                     Si %       54       41        40   63    93                                   cracking % 46       59        60   37    7                                    Isomerisation                                                                            402      73        672  595   106                                  output                                                                        (r × Si)                                                                Isomers in C6 %                                                               DM-2,2B    --       --         1   --    --                                   M2P        --       34        28   57    15                                   M3P        --        8        23   24    25                                   HEX         1       10        12   15    17                                   MCP        --       --        --   --    --                                   BEN        99       48        35    4    26                                   CYC        --       --        --   --    17                                   Hydrocarbons %                                                                C5 + C1    44       20        20   26    68                                   C4 + C2    20       22        21   20    5                                    2C3        21       16        15   13    5                                    3C2         3       12        12    6    22                                   6Cl        12       30        32   34    --                                   ______________________________________                                         *Reactions carried out with different catalyst masses                    

                  TABLE 6a                                                        ______________________________________                                        Isomeriston of n-hexane on catalyst WC                                        reactivated in various ways.                                                                       Invention                                                        Prior art    with        Al2O3                                        Reactivation of                                                                         with H2  with H2 + H2 + pentane                                                                            0.25%                                  the carbide                                                                             alone    pentane   0,05% Pt. de Pt.                                 ______________________________________                                        Conversion                                                                              600      450       450   350   350                                  temperature %                                                                 Conversion %                                                                            32,05    3,78      16,05*                                                                              15,82*                                                                              2,88                                 r. (10 exp -10                                                                          1336     314       3510  1317  120                                  mole/s.g)                                                                     Si %      13       35        18    17    90                                   cracking %                                                                              87       65        82    83    10                                   Isomerisation                                                                           174      110       632   224   108                                  output                                                                        (r × Si)                                                                Isomers                                                                       in C6 %                                                                       DM-2,2B   --       --        --    --    --                                   M2P       26       28        23    46    54                                   M3P       10       19        14    19    38                                   HEX       --       --        -     --    --                                   MCP       10       30        14    25    8                                    BEN       54        3        49    10    --                                   CYC       --       20        --    --    --                                   Hydro-                                                                        carbides %                                                                    C5 + C1    6       27        17    27    71                                   C4 + C2   18       31        24    25    18                                   2C3       28       21        21    22    11                                   3C2       24        9        13     6    --                                   6Cl       23       12        25    20    --                                   ______________________________________                                         *Reactions carried out with different catalyst masses                    

Example 5

This example illustrates the application of carbides with a highspecific surface area to the conversion of exhaust gases.

An Mo₂ C carbide was used in the form of a granular material of 0.2 to0.5 mm having a specific surface area of 168 sq.m/g prepared by themethod illustrated in Example 1 and activated at 600° C. by the methodof the second improvement, using 0.05% Pt, a flow of hydrogen andpropane and a finishing treatment under H₂ at 400° C. for 12 hours andthen 600° C. for 2 hours.

Then, taking as a pattern the combustion of various pure mixturescomposed of light petrol and air (mixtures: rich in petrol,stoichiometric, lean), various exhaust gases were then passed through400 mg of granular carbide at the rate of 250 cc/min.

Table 7 summarises the experimental conditions and the conversionresults obtained; by way of comparison, Test 4 relates to astoichiometric fuel mixture, of which the exhaust gases were convertedby means of one and the same conventional catalyst mass (400 mg)consisting of an Al₂ O₃ carrier with a large surface area and 1% Pt and0.2% Rh as active elements.

                                      TABLE 7                                     __________________________________________________________________________                        Composition of                                                                exhaust gases                                                                         Maximum semi-                                               Richness of the                                                                         prior to                                                                              conversion                                                                            Maximum                                             fuel mixture                                                                            conversion                                                                            temperature                                                                           conversion                                          (Volume of air per                                                                      ppm volume                                                                            (light off)                                                                           Temperature                                                                          Rate                                         1 vol. of petrol)                                                                       (balance = N2)                                                                        °C.                                                                            °C.                                                                           %                                  __________________________________________________________________________    Test 1                                                                        Rich mixture                                                                            14,25                                                               CO                  9700    380     400    20                                 NO                  2050    450     600    100                                O2                  2400            400    100                                Hydrocarbon (HC)     950    400     600    40                                 Test 2                                                                        Stoichiometric                                                                          14,75                                                               mixture                                                                       CO                  5700    350     450    90                                 NO                  2050    400     470    70                                 O2                  6200            400    100                                HC                   950    380     600    75                                 Test 3                                                                        Lean mixture                                                                            15,25                                                               CO                  1800    450     450    90                                 NO                  2050    400     450    15                                 O2                  10000           500    70                                 HC                   950    400     500    65                                 Test 4                                                                        Stoichiometric                                                                          14,75                                                               mixture                                                                       CO                  5700    250     400    100                                NO                  2050    250     500    80                                 O2                  6200                                                      HC                   950    250     500    70                                 __________________________________________________________________________

It can be seen that although the semi-conversion temperatures are higherthan those obtained with the conventional catalyst, the conversion ratesare comparable. The fact that it is necessary to use a rather higherconversion temperature with the carbides is no problem; that istantamount to bringing the catalytic converter closer to the source ofthe exhaust gases, which is possible. On the other hand, the cost of thecatalyst according to the invention is clearly advantageous.

Example 6

This example illustrates the application of carbides according to theinvention to the catalysis of chemical reactions. Here, this will be theproduction of benzene (BEN) by catalytic and selective dehydrogenationof the cyclohexane (CYC).

This reaction is carried out at 450° C. in the presence of variouscatalysts: a molybdenum carbide with a high specific surface areaaccording to the invention, the same carbide but this time activated byvarious metals of group 8 according to the second improvement, and aconventional catalyst based on 0.25% Pt deposited on alumina carrier.

Table 8 shows the overall results; it gives the percentage of CYCconverted and the selectivity of the conversion, that is to say theproportion in percent of the quantity of benzene obtained from among theconverted products, the efficiency of the conversion appreciating by theconversion-selectivity product. In the catalyst column is noted theresidual content of the metal used to carry out the activationtreatment.

It is more important to have good selectivity than good output or a goodrate of conversion; indeed, the by-products of cracking are uselesswhile with a very high level of selectivity and insufficient output itis possible easily to recycle the non-converted CYC to produce BEN.

                  TABLE 8                                                         ______________________________________                                                     CON-      SELECTIVITY                                                         VERSION   FOR BENZENE  OUT-                                      CATALYST     (%)       (%)          PUT                                       ______________________________________                                        Mo2C          6        100          0.06                                      Mo2C + 690 ppm Rh                                                                          76        97           0.74                                      Mo2C + 500 ppm Pd                                                                          98        73           0,71                                      Mo2C + 500 ppm Co                                                                          69        66           0,46                                      Pt (0,25%)/alumina                                                                         100       61           0,61                                      ______________________________________                                    

It can be seen that the molybdenum carbide according to the inventiondisplays very good selectivity; on the other hand, its conversionefficiency needs to be improved by an activation treatment according tothe invention without adversely affecting its selectivity and then theresult is an output or efficiency which is generally better than thatobtained with a conventional catalyst based on Pt on an alumina carrier.Activation by means of Co can be interesting on account of the verymoderate cost of the said Co.

What is claimed is:
 1. A method of producing a composite carbide of highspecific surface area, comprising reacting a compound of a heavy metalin a gaseous state with reactive carbon having a specific surface areaat least equal to 200 sq.m/g at a temperature between 900° C. and 1400°C., limiting the conversion of reactive carbon into carbide to obtain aproduct comprising a core of reactive carbon and an outer layer ofmetallic carbide with a specific surface area at least 20 m² /g,andtreating said product to convert said core to silicon carbide or aheavy metal carbide different from said outer layer.
 2. A methodaccording to claim 1, wherein said product is treated at a temperaturebetween 1100° and 1400° C. in the presence of gaseous SiO as to convertthe core to silicon carbide.
 3. A method according to claim 1, whereinsaid product is treated at a temperature between 900° and 1400° C. witha compound in the gaseous state of a heavy metal other than that used toproduce said outer layer, to convert the core to a heavy metal carbide.4. A method of producing a heavy metal carbide of specific surface areaof at least 20 m² /g, comprising reacting a compound of said heavy metalin a gaseous state with reactive carbon having a specific surface areaat least equal to 200 m² /g at a temperature between 900° C. and 1400°C., to produce a heavy metal carbide, impregnating said carbide with asolution of a soluble salt of at least one group 8 metal to obtain ametal impregnated carbide, drying said impregnated carbide and treatingsaid dried and impregnated carbide at a temperature of at least 500° C.under a stream of hydrogen and gaseous hydrocarbon.
 5. A methodaccording to claim 4, wherein the group 8 metal is selected from thegroup consisting of Pt, Pd, Rh, Ir, Co, Ni, and Fe.
 6. A methodaccording to claim 5, wherein the metal is Pt and the quantity of Ptretained by the carbide is at most about 0.05% based on the weight of Ptcompared with the carbide.
 7. A method according to any one of claims 4to 6, wherein said treatment of dried and impregnated carbide is carriedout with a mixture of H₂ and gaseous hydrocarbon in excess, containingfrom 1 to 50%, by volume, hydrocarbon, the excess of said mixture beingrecycled after elimination of a by-product which is reaction water.
 8. amethod according to any one of claims 4 to 6, wherein the hydrocarbon isan aliphatic hydrocarbon.
 9. A method according to any one of claims 4to 6, additionally comprising a finishing treatment comprising passingover the heat treated carbide a stream of hydrogen at a temperaturebetween 300° and 700° C. for 1 to 20 hours, said finishing treatmentoptionally preceded by the establishment of a vacuum at around 350° to500° C. for 1 to 3 hrs.
 10. A mixed carbide obtained according to claim1, having a specific surface area of at least 20 m² /g, and comprisingan outer layer of heavy metal carbide and a core of silicon carbide or acarbide of a heavy metal different from said outer layer.
 11. A mixedcarbide according to claim 10, wherein the outer layer comprisesmolybdenum carbide and the core is silicon carbide, and the outer layeradditionally comprises molybdenum silicide.
 12. A carbide obtainedaccording to claim 1, having an outer layer of a hexagonal Mo₂ C carbidewith a specific surface area greater than 200 sq.m/g.
 13. A carbideobtained according to claim 1, having an outer layer of a hexagonal WCcarbide with a specific surface area greater than 300 sq.m/g.